Optical cross-coupling mitigation systems for wavelength beam combining laser systems

ABSTRACT

In various embodiments, wavelength beam combining laser systems incorporate optical cross-coupling mitigation systems and/or engineered partially reflective output couplers in order to reduce or substantially eliminate unwanted back-reflection of stray light.

RELATED APPLICATIONS

This application (A) claims the benefit of and priority to U.S.Provisional Patent Application No. 62/089,839, filed on Dec. 10, 2014,and (B) is a continuation-in-part of U.S. patent application Ser. No.14/746,951, filed on Jun. 23, 2015, which is a continuation of U.S.patent application Ser. No. 13/841,821, filed on Mar. 15, 2013, which isa continuation-in-part of U.S. patent application Ser. No. 13/218,251,filed on Aug. 25, 2011, which (i) claims the benefit of and priority toU.S. Provisional Patent Application No. 61/376,900, filed on Aug. 25,2010, and (ii) is a continuation-in-part of U.S. patent application Ser.No. 13/042,042, filed on Mar. 7, 2011, which claims the benefit of andpriority to U.S. Provisional Patent Application No. 61/310,777, filed onMar. 5, 2010, U.S. Provisional Patent Application No. 61/310,781, filedon Mar. 5, 2010, and U.S. Provisional Patent Application No. 61/417,394,filed on Nov. 26, 2010. The entire disclosure of each of theseapplications is hereby incorporated herein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to laser systems,specifically wavelength beam combining laser systems incorporatingsystems for mitigating optical cross-coupling between beam emitters.

BACKGROUND

High-power laser systems are utilized for a host of differentapplications, such as welding, cutting, drilling, and materialsprocessing. Such laser systems typically include a laser emitter, thelaser light from which is coupled into an optical fiber (or simply a“fiber”), and an optical system that focuses the laser light from thefiber onto the workpiece to be processed. The optical system istypically engineered to produce the highest-quality laser beam, or,equivalently, the beam with the lowest beam parameter product (BPP). TheBPP is the product of the laser beam's divergence angle (half-angle) andthe radius of the beam at its narrowest point (i.e., the beam waist, theminimum spot size). The BPP quantifies the quality of the laser beam andhow well it can be focused to a small spot, and is typically expressedin units of millimeter-milliradians (mm-mrad). A Gaussian beam has thelowest possible BPP, given by the wavelength of the laser light dividedby pi. The ratio of the BPP of an actual beam to that of an idealGaussian beam at the same wavelength is denoted M², or the “beam qualityfactor,” which is a wavelength-independent measure of beam quality, withthe “best” quality corresponding to the “lowest” beam quality factor of1.

Wavelength beam combining (WBC) is a technique for scaling the outputpower and brightness from laser diode bars, stacks of diode bars, orother lasers arranged in a one- or two-dimensional array. WBC methodshave been developed to combine beams along one or both dimensions of anarray of emitters. Typical WBC systems include a plurality of emitters,such as one or more diode bars, that are combined using a dispersiveelement to form a multi-wavelength beam. Each emitter in the WBC systemindividually resonates, and is stabilized through wavelength-specificfeedback from a common partially reflecting output coupler that isfiltered by the dispersive element along a beam-combining dimension.Exemplary WBC systems are detailed in U.S. Pat. No. 6,192,062, filed onFeb. 4, 2000, U.S. Pat. No. 6,208,679, filed on Sep. 8, 1998, U.S. Pat.No. 8,670,180, filed on Aug. 25, 2011, and U.S. Pat. No. 8,559,107,filed on Mar. 7, 2011, the entire disclosure of each of which isincorporated by reference herein.

A variety of WBC techniques have been utilized to form high-power lasersfor many different applications. However, optical cross-coupling betweenbeam emitters may result in conventional WBC systems having sub-optimalbrightness. Thus, there is a need for cross-coupling mitigationarrangements for WBC laser systems.

SUMMARY

In accordance with embodiments of the present invention, wavelength beamcombining (WBC) laser systems feature multiple emitters (or “beamemitters”), e.g., diode bars or the individual diode emitters of a diodebar, which are combined to form a multi-wavelength beam. Each emitter inthe laser system individually resonates and is stabilized viawavelength-specific feedback from a common partially reflecting outputcoupler that is filtered by a dispersive element (e.g., a diffractiongrating, a dispersive prism, a grism (prism/grating), a transmissiongrating, or an Echelle grating) along a beam-combining dimension.Advantageously, cross-talk between feedback beams is mitigated using anon-slit-based cross-coupling mitigating optical system. In variousembodiments, the cross-coupling mitigation system, or at least a portionthereof, is positioned within the Rayleigh range of the multi-wavelengthbeam transmitted by the dispersive element, and the output coupler ispositioned within the Rayleigh range of the multi-wavelength beamtransmitted by the cross-coupling mitigation system (or at least aportion thereof). In this manner, laser systems in accordance withembodiments of the present invention produce multi-wavelength outputbeams having high brightness and high power.

In various embodiments, the cross-coupling mitigation system includes orconsists essentially of first and second optical elements (e.g.,lenses), and the focal length of the first optical element is greater(or even substantially greater) than the focal length of the secondoptical element. In such embodiments, the first optical element may bepositioned within the Rayleigh range of the multi-wavelength beamtransmitted by the dispersive element, and the output coupler may bepositioned within the Rayleigh range of the multi-wavelength beamtransmitted by the second optical element.

In various embodiments, optical cross-coupling is also reduced orsubstantially eliminated via the use of engineered output couplers thatminimize back-reflection of stray wavelengths that might reflect back tothe individual beam emitters. Such output couplers may be utilized withor without other cross-coupling mitigation systems described herein. Invarious embodiments, the partially reflective output couplerincorporates an anti-reflection coating on its surface in regions otherthan a partially reflective portion sized and positioned to interceptonly the multi-wavelength beam. The partially reflective portion mayprotrude from the remaining portion of the output coupler, or thepartially reflective portion may be substantially coplanar with theremaining portion.

In various embodiments of the invention, the output coupler may includeor consist essentially of an optical fiber, the core of which is sizedand positioned to intercept only the multi-wavelength beam. The surfaceof the core may be partially reflective, and/or the core may includetherewithin a fiber Bragg grating to provide the feedback-enablingreflection of the beam. The cladding of the optical fiber may be coatedwith an anti-reflection coating to prevent stray reflection and opticalcross-coupling resulting therefrom. An end cap may be present over theoptical fiber for, e.g., environmental protection and/or to reduce thepower density at the end of the fiber. In various embodiments, theoptical fiber may incorporate and/or be utilized in conjunction with amode stripper that substantially eliminates unwanted modes of light frompropagating within the optical fiber.

Embodiments of the present invention couple multi-wavelength outputbeams into an optical fiber. In various embodiments, the optical fiberhas multiple cladding layers surrounding a single core, multiplediscrete core regions (or “cores”) within a single cladding layer, ormultiple cores surrounded by multiple cladding layers. In variousembodiments, the output beams may be delivered to a workpiece forapplications such as cutting, welding, etc.

Herein, “optical elements” may refer to any of lenses, mirrors, prisms,gratings, and the like, which redirect, reflect, bend, or in any othermanner optically manipulate electromagnetic radiation. Herein, beamemitters, emitters, or laser emitters, or lasers include anyelectromagnetic beam-generating device such as semiconductor elements,which generate an electromagnetic beam, but may or may not beself-resonating. These also include fiber lasers, disk lasers, non-solidstate lasers, vertical cavity surface emitting lasers (VCSELs), etc.Generally, each emitter includes a back reflective surface, at least oneoptical gain medium, and a front reflective surface. The optical gainmedium increases the gain of electromagnetic radiation that is notlimited to any particular portion of the electromagnetic spectrum, butthat may be visible, infrared, and/or ultraviolet light. An emitter mayinclude or consist essentially of multiple beam emitters such as a diodebar configured to emit multiple beams. The input beams received in theembodiments herein may be single-wavelength or multi-wavelength beamscombined using various techniques known in the art.

Laser diode arrays, bars and/or stacks, such as those described in thefollowing general description may be used in association withembodiments of the innovations described herein. Laser diodes may bepackaged individually or in groups, generally in one-dimensionalrows/arrays (diode bars) or two dimensional arrays (diode-bar stacks). Adiode array stack is generally a vertical stack of diode bars. Laserdiode bars or arrays generally achieve substantially higher power, andcost effectiveness than an equivalent single broad area diode.High-power diode bars generally contain an array of broad-area emitters,generating tens of watts with relatively poor beam quality; despite thehigher power, the brightness is often lower than that of a broad arealaser diode. High-power diode bars may be stacked to produce high-powerstacked diode bars for generation of extremely high powers of hundredsor thousands of watts. Laser diode arrays may be configured to emit abeam into free space or into a fiber. Fiber-coupled diode-laser arraysmay be conveniently used as a pumping source for fiber lasers and fiberamplifiers.

A diode-laser bar is a type of semiconductor laser containing aone-dimensional array of broad-area emitters or alternatively containingsub arrays containing, e.g., 10-20 narrow stripe emitters. A broad-areadiode bar typically contains, for example, 19-49 emitters, each havingdimensions on the order of, e.g., 1 μm×100 μm. The beam quality alongthe 1 μm dimension or fast-axis is typically diffraction-limited. Thebeam quality along the 100 μm dimension or slow-axis or array dimensionis typically many times diffraction-limited. Typically, a diode bar forcommercial applications has a laser resonator length of the order of 1to 4 mm, is about 10 mm wide and generates tens of watts of outputpower. Most diode bars operate in the wavelength region from 780 to 1070nm, with the wavelengths of 808 nm (for pumping neodymium lasers) and940 nm (for pumping Yb:YAG) being most prominent. The wavelength rangeof 915-976 nm is used for pumping erbium-doped or ytterbium-dopedhigh-power fiber lasers and amplifiers.

A diode stack is simply an arrangement of multiple diode bars that candeliver very high output power. Also called diode laser stack, multi-barmodule, or two-dimensional laser array, the most common diode stackarrangement is that of a vertical stack which is effectively atwo-dimensional array of edge emitters. Such a stack may be fabricatedby attaching diode bars to thin heat sinks and stacking these assembliesso as to obtain a periodic array of diode bars and heat sinks. There arealso horizontal diode stacks, and two-dimensional stacks. For the highbeam quality, the diode bars generally should be as close to each otheras possible. On the other hand, efficient cooling requires some minimumthickness of the heat sinks mounted between the bars. This tradeoff ofdiode bar spacing results in beam quality of a diode stack in thevertical direction (and subsequently its brightness) is much lower thanthat of a single diode bar. There are, however, several techniques forsignificantly mitigating this problem, e.g., by spatial interleaving ofthe outputs of different diode stacks, by polarization coupling, or bywavelength multiplexing. Various types of high-power beam shapers andrelated devices have been developed for such purposes. Diode stacks mayprovide extremely high output powers (e.g. hundreds or thousands ofwatts).

In an aspect, embodiments of the invention feature a laser system thatincludes or consists essentially of an array (e.g., a one-dimensionalarray or a two-dimensional array) of beam emitters each emitting a beam,focusing optics for focusing the beams toward a dispersive element, adispersive element for receiving and dispersing the focused beams,thereby forming a multi-wavelength beam, and an optical fiber forreceiving the multi-wavelength beam. The optical fiber includes orconsists essentially of (i) a core for receiving the multi-wavelengthbeam, reflecting a first portion thereof back toward the dispersiveelement, and transmitting a second portion thereof as an output beamcomposed of multiple wavelengths, the core having a partially reflectivesurface, and (ii) surrounding the core, a cladding having a reflectivityto the multi-wavelength beam of less than 1%.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. A portion of the core may protrudefrom the cladding. A surface of the core may be substantially coplanarwith a surface of the cladding. The optical fiber may be positioned suchthat, at the partially reflective surface of the core, a diameter (orother lateral dimension, e.g., width) of the core is not less than adiameter (or other lateral dimension, e.g., width) of themulti-wavelength beam. The diameter of the core may be substantiallyequal to or greater than the diameter of the multi-wavelength beam. Anend cap may be attached to the optical fiber and disposed opticallyupstream of the partially reflective surface of the core. Ananti-reflective coating may be disposed over the cladding of the opticalfiber. A mode stripper may be disposed around at least a portion of thecore of the optical fiber. The mode stripper may be disposed around atleast a portion of the cladding of the optical fiber. The focusingoptics may include or consist essentially of one or more cylindricallenses, one or more spherical lenses, one or more spherical mirrors,and/or one or more cylindrical mirrors. The dispersive element mayinclude or consist essentially of a diffraction grating (e.g., atransmissive diffraction grating or a reflective diffraction grating).

The laser system may include a cross-coupling mitigation system forreceiving and transmitting the multi-wavelength beam while reducingcross-coupling thereof. The partially reflecting surface of the core ofthe optical fiber may be disposed within a Rayleigh range of themulti-wavelength beam transmitted by the cross-coupling mitigationsystem. At least a portion of the cross-coupling mitigation system maybe disposed within a Rayleigh range of the multi-wavelength beamtransmitted by the dispersive element. The cross-coupling mitigationsystem may be afocal. The cross-coupling mitigation system may includeor consist essentially of an afocal telescope. The cross-couplingmitigation system may include or consist essentially of a first opticalelement having a first focal length and a second optical element havinga second focal length. The first optical element may be disposedoptically upstream of the second optical element. The first focal lengthmay be at least two, at least three, at least five, at least seven, atleast ten, or at least 100 times greater than the second focal length.Each of the first and second optical elements may include or consistessentially of a lens (e.g., a cylindrical lens or a spherical lens).The first optical element may be disposed within a Rayleigh range of themulti-wavelength beam transmitted by the dispersive element. Thepartially reflecting surface of the core of the optical fiber may bedisposed within a Rayleigh range of the multi-wavelength beamtransmitted by the second optical element. The optical distance betweenthe first and second optical elements may be approximately equal to asum of the first and second focal lengths.

In another aspect, embodiments of the invention feature a laser systemthat includes or consists essentially of an array of beam emitters eachemitting a beam, focusing optics for focusing the beams toward adispersive element, a dispersive element for receiving and dispersingthe focused beams, thereby forming a multi-wavelength beam, across-coupling mitigation system for receiving and transmitting themulti-wavelength beam while reducing cross-coupling thereof, disposedoptically downstream of the cross-coupling mitigation system, an opticalfiber for receiving the multi-wavelength beam, and disposed within theoptical fiber, a fiber Bragg grating for receiving the multi-wavelengthbeam, reflecting a first portion thereof back toward the cross-couplingmitigation system, and transmitting a second portion thereof as anoutput beam composed of multiple wavelengths.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. An end cap may be attached to theoptical fiber and disposed optically upstream of the fiber Bragggrating. The focusing optics may include or consist essentially of oneor more cylindrical lenses, one or more spherical lenses, one or morespherical mirrors, and/or one or more cylindrical mirrors. Thedispersive element may include or consist essentially of a diffractiongrating (e.g., a transmissive diffraction grating or a reflectivediffraction grating). A mode stripper may be disposed around at least aportion of the optical fiber. The fiber Bragg grating may be disposedwithin a Rayleigh range of the multi-wavelength beam transmitted by thecross-coupling mitigation system. At least a portion of thecross-coupling mitigation system may be disposed within a Rayleigh rangeof the multi-wavelength beam transmitted by the dispersive element. Thecross-coupling mitigation system may be afocal. The cross-couplingmitigation system may include or consist essentially of an afocaltelescope. The cross-coupling mitigation system may include or consistessentially of a first optical element having a first focal length and asecond optical element having a second focal length. The first opticalelement may be disposed optically upstream of the second opticalelement. The first focal length may be at least two, at least three, atleast five, at least seven, at least ten, or at least 100 times greaterthan the second focal length. Each of the first and second opticalelements may include or consist essentially of a lens (e.g., acylindrical lens or a spherical lens). The first optical element may bedisposed within a Rayleigh range of the multi-wavelength beamtransmitted by the dispersive element. The fiber Bragg grating may bedisposed within a Rayleigh range of the multi-wavelength beamtransmitted by the second optical element. The optical distance betweenthe first and second optical elements may be approximately equal to asum of the first and second focal lengths.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “substantially” and “approximately” mean±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein. Nonetheless, such other materials may be present, collectivelyor individually, in trace amounts. Herein, the terms “radiation” and“light” are utilized interchangeably unless otherwise indicated. Herein,“downstream” or “optically downstream,” is utilized to indicate therelative placement of a second element that a light beam strikes afterencountering a first element, the first element being “upstream,” or“optically upstream” of the second element. Herein, “optical distance”between two components is the distance between two components that isactually traveled by light beams; the optical distance may be, but isnot necessarily, equal to the physical distance between two componentsdue to, e.g., reflections from mirrors or other changes in propagationdirection experienced by the light traveling from one of the componentsto the other.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A is a schematic of a wavelength beam combining (WBC) method inthe non-beam-combining dimension in accordance with embodiments of theinvention;

FIG. 1B is a schematic of a wavelength beam combining (WBC) method inthe beam-combining dimension in accordance with embodiments of theinvention; FIG. 2 is a schematic of a WBC laser system that incorporatesan optical cross-coupling mitigation system in accordance withembodiments of the invention;

FIG. 3 is a schematic of an exemplary optical cross-coupling mitigationsystem for a WBC laser system in accordance with embodiments of theinvention;

FIG. 4 is a schematic of an optical cross-coupling mitigation system andan output coupler for a WBC laser system in accordance with embodimentsof the invention;

FIG. 5 is a schematic of an optical element and an output coupler for aWBC laser system in accordance with embodiments of the invention; and

FIGS. 6-8 are schematics of portions of optical fibers utilized asoutput couplers for WBC laser systems in accordance with embodiments ofthe invention.

DETAILED DESCRIPTION

Aspects and embodiments relate generally to the field of scaling lasersources to high-power and high-brightness using an external cavity and,more particularly, to methods and apparatus for external-cavity beamcombining using both one-dimensional or two-dimensional laser sources.In one embodiment the external cavity system includes one-dimensional ortwo-dimensional laser elements, an optical system, a dispersive element,and a partially reflecting element. An optical system is one or moreoptical elements that perform two basic functions. The first function isto overlap all the laser elements along the beam combining dimensiononto a dispersive element. The second function is to ensure all theelements along the non-beam combining dimension are propagating normalto the output coupler. In various embodiments, the optical systemintroduces as little loss as possible. As such, these two functions willenable a single resonance cavity for all the laser elements.

In another embodiment the WBC external cavity system includes wavelengthstabilized one-dimensional or two-dimensional laser elements, an opticalsystem, and a dispersive element. One-dimensional or two-dimensionalwavelength stabilized laser elements, with unique wavelength, can beaccomplished using various means such as laser elements with feedbackfrom wavelength chirped Volume Bragg grating, distributed feedback (DFB)laser elements, or distributed Bragg reflector (DBR) laser elements.Here the main function of the optical system is to overlap all the beamsonto a dispersive element. When there is no output coupler mirrorexternal to the wavelength-stabilized laser element, having parallelbeams along the non-beam-combining dimension is less important. Aspectsand embodiments further relate to high-power and/or high-brightnessmulti-wavelength external-cavity lasers that generate an overlapping orcoaxial beam from very low output power to hundreds and even tomegawatts of output power.

Embodiments of the present invention mitigate the amount of unintendedand/or undesired feedback from non-originated emitters in WBC lasersystems. For example, in a WBC system in which two individual beamemitters share a common partially-reflective mirror (such as an outputcoupler), there is the potential for feedback light from one emitter toenter the other emitter. This undesirable feedback (or “cross-talk” or“cross-coupling”) from a “non-originated” emitter reduces the efficiencyof the system. The approaches and embodiments described herein may applyto one- and two-dimensional beam combining systems along theslow-diverging dimension (or “direction”), fast-diverging dimension, orother beam combining dimensions. For purposes of this application,emitted beams have profiles in which one dimension is close to or fullydiffraction limited, while the other dimension is many times diffractionlimited. Another way of describing this may be in terms of axis and/ordimension. For example, an output beam may have a slow and a fastdiverging axis or dimension.

When using the term substantially greater, when referring to the focallength of one optical element as compared to the focal length of anotheroptical element (f1>>f2), it is to be understood that to be a factor ofat least 2, 3, 4, 5, 7 times or greater. For example, the focal lengthof f1 may be 100 mm or more while the focal length of f2 is 50 mm orless. In another example, the focal length of f1 may be 200 mm or morewhile f2 is 20 mm or less. The term “angular filter” refers to aplurality of optical elements that create a specified numerical aperturefor feedback beams. The size of this numerical aperture may limit theallowed feedback to only that corresponding to the originally emittedbeam. That is, the angular filter prevents adjacent or nearby emittedbeams from returning into the original emitter (i.e., cross-talk).Stabilization of emitters refers to feedback received by each emitterthat has been narrowed to a distinct wavelength. This may be in the formof seeding the emitters with a particular wavelength, causing a portionof the emitted beam to be redirected back into the emitter, andintervening with the feedback, such as placing an optical grating in theway, to produce a distinct wavelength to be directed into the emittersas feedback. Often, feedback is reflected back towards the originalemission area, where it passes through a dispersive element ordiffraction grating prior to entering back into the optical gain mediumportion of the original emitter. In some WBC embodiments, the feedbacksource may be a common reflective surface that provides feedback tomultiple emitters, with each of the feedback beams being individuallytuned to a particular wavelength.

FIGS. 1A-1B illustrate an external-cavity one-dimensional (1-D) WBCsystem including or consisting essentially of a one-dimensional beamemitter 102 (e.g., a diode bar) having a back reflective surface 104, again medium 106 with, e.g., two or more diode emitters 105, a frontreflective surface 108, a combining optic 110, a dispersive element 112,and a partially reflecting output coupler 114. In this embodiment, thecombining optic or lens 110 is placed a focal distance 120 a away fromthe front reflective surface 108 of the diode bar 102 while on the backplane or other side of lens 110, dispersive element 112 is placed afocal distance 120 b away. The output coupler 114 is placed at a certaindistance from the dispersive element 112 and reflects a portion of thegenerated beams (feedback 116) back towards dispersive element 112.

In this embodiment, the placement of the combining lens 110 accomplishestwo functions. The first function is to overlap all the chief rays fromall the diode elements onto the dispersive element 112. The secondfunction is to collimate each beam in both axes. FIGS. 1A and 1Billustrate a schematic view of the non-beam-combining dimension 130 view(FIG. 1A) and the beam-combining dimension 140 view (FIG. 1B). Emitter102 includes or consists essentially of multiple emitters (e.g., diodeemitters) 105, a back reflecting surface 104, gain medium 106, and afront surface/facet 108.

In WBC resonators it is possible for adjoining emitters to opticallycross-couple with each other. This may seriously degrade the output beamquality. FIG. 2 is a schematic of a WBC resonator with two adjoiningemitters 202 a and 202 b sending their nominal on-axis chief rays 260 aand 260 b (shown as solid lines) to lens 210, which focuses them ontothe center of the dispersive element (e.g., diffraction grating) 212.From there, both chief rays are diffracted at their own uniquewavelengths to propagate along the same axis 240, through thecross-coupling mitigation optics 250 that represents any and all lensesor optical elements between the grating 212 and the partially-reflectivecoupler 214. Both rays are then partially reflected back ontothemselves, propagating backwards to self-couple into their respectiveemitters. The dashed lines 261 a and 261 b in FIG. 2 show the chief raysthat would result in optical cross-coupling between the twoemitters—i.e. a chief ray emanating from one emitter couples back intoanother emitter.

The following parameters are defined as follows:

d=distance between the two emitters (symmetrically displaced above andbelow the axis by+/−(d/2)).

ε=deviation angle (the angle between the solid-line chief rays and thedotted-line chief rays at the emitters.

θ_(1/2)=semi-divergence far-field angle of an emitter in the WBCdirection.

L₀=distance from the emitters to lens L1.

f₁=focal length of lens L1.

In FIG. 2, the grating is shown as if it worked at normal incidence.Here, it is assumed that the system is operating in the Littrowconfiguration, where the incident angle and the diffracted angle areequal (and non-zero). In the Littrow configuration, a small change inthe incident angle is matched to first order by an equal change in thediffracted angle. In the unfolded schematic, then, any ray operating atLittrow would appear to propagate straight through the grating. It isclear that only the center ray 202 c (the one that would emanate from animaginary emitter halfway between two emitters 202 a and 202 b)self-couples at Littrow.

The symmetry in FIG. 2 is deliberate, as it allows for a couple ofimportant simplifications in the analysis of the unique deviation angleε at which a chief ray could exit one emitter and return to the other.The first symmetry-based simplification is that the deviated(dashed-line) chief ray must hit the coupler at its center. The secondsimplification is that the cross-coupling wavelength must be the averageof the two self-coupling wavelengths. This would in turn be thewavelength of an imaginary emitter halfway between the two emitters,which, as noted above, would self-couple at Littrow. Therefore, thedashed-line chief rays in FIG. 2 must traverse the grating at Littrow,meaning that they would appear to propagate straight through the gratingas shown. Using this simplification, a conventional “y/y-bar” (chief rayheight/chief ray slope) analysis may be utilized to trace the topdashed-line chief ray:

Leaving the top emitter:

$\begin{matrix}{y_{emitter} = \frac{d}{2}} & (1) \\{{\overset{\_}{y}}_{emitter} = ɛ} & (2)\end{matrix}$

Entering lens L1:

$\begin{matrix}{y_{L\; 1{\_{in}}} = {{y_{emitter} + {L_{0}{\overset{\_}{y}}_{emitter}}} = {\frac{d}{2} + {L_{0}\varepsilon}}}} & (3) \\{{\overset{\_}{y}}_{L\; 1{\_{in}}} = {{\overset{\_}{y}}_{emitter} = ɛ}} & (4)\end{matrix}$

Exiting lens L1:

$\begin{matrix}{y_{L\; 1{\_{out}}} = {{\overset{\_}{y}}_{L\; 1{\_{in}}} = {\frac{d}{2} + {L_{0}\varepsilon}}}} & (5) \\{{\overset{\_}{y}}_{L\; 1_{out}} = {{{\overset{\_}{y}}_{L\; 1_{in}} - \frac{y_{L\; 1_{in}}}{f_{1}}} = {{ɛ - \frac{\frac{d}{2} + {L_{0}\varepsilon}}{f_{1}}} = {{ɛ\left( \frac{f_{1} - L_{0}}{f_{1}} \right)} - \frac{d}{2\; f_{1}}}}}} & (6)\end{matrix}$

Entering and exiting the grating (recall from the discussion above thatthe dashed-line chief ray does not change direction at the grating):

$\begin{matrix}{y_{grating} = {{y_{L\; 1{\_{out}}} + {f_{1}{\overset{\_}{y}}_{L\; 1{\_{out}}}}} = {{\frac{d}{2} + {L_{0}\varepsilon} + {\varepsilon\left( {f_{1} - L_{0}} \right)} - \frac{d}{2}} = {ɛ\; f_{1}}}}} & (7) \\{{\overset{\_}{y}}_{grating} = {{\overset{\_}{y}}_{L\; 1{\_{out}}} = {{ɛ\left( \frac{f_{1} - L_{0}}{f_{1}} \right)} - \frac{d}{2\; f_{1}}}}} & (8)\end{matrix}$

To finish the calculations at the coupler, which involves propagatingthrough the cross-coupling mitigation optics, recall that thedashed-line chief ray intersects the coupler at its center. Therefore,only the ray slope at the coupler is non-zero, and note that the rayheight and ray slope at the grating must both be proportional to the rayslope at the coupler. This implies that the ratio of the height to theslope at the grating must be a constant. And, one may interpret thatconstant very intuitively as the negative of the effective distance ofthe coupler from the grating, as determined by the cross-couplingmitigation optics. In other words,

$\begin{matrix}{\frac{y_{grating}}{{\overset{\_}{y}}_{grating}} \equiv {- L_{{cplr}\;\_\;{eff}}}} & (9)\end{matrix}$Where L_(cplr) _(_) _(eff) is the effective distance of the couplerbeyond (to the right of) the grating.

In practice, L_(cplr eff) can be calculated either with a raytrace orwith a y/y-bar analysis of the post-grating lenses. But in any case,Equation 9 allows one to solve Equations 7 and 8 for the deviation angleε with the following result:

$\begin{matrix}{ɛ = {\left( \frac{d}{2\; f_{1}} \right)/\left\lbrack {\left( \frac{f_{1} - L_{0}}{f_{1}} \right) + \left( \frac{f_{1}}{L_{{cplr}\;\_\;{eff}}} \right)} \right\rbrack}} & (10)\end{matrix}$

Now that the deviation angle ε that results in cross-coupling has beendetermined, the amount of cross-coupling may be calculated. Onereasonable definition of the cross-coupling is the integral over solidangle at the emitter of the product of the self-coupled intensity andthe cross-coupled intensity, normalized by the integral of the square ofthe self-coupled intensity. Prior to calculating that integral, it isimportant to note that in the name of simplifying symmetry, it isassumed that both the outgoing and incoming beams at the cross-couplingemitters equally deviate. Thus, for the overlap integral, consider onebeam (the self-coupled beam) to be undeviated, and the other beam (thecross-coupled beam) to be deviated by twice the angle ε. Putting thisparagraph into equation form provides:

$\begin{matrix}{{overlap} = \frac{\int{{\exp\left\lbrack {{- 2}\left( \frac{\theta}{\theta_{1/2}} \right)^{2}} \right\rbrack}{\exp\left\lbrack {{- 2}\left( \frac{\theta - {2ɛ}}{\theta_{1/2}} \right)^{2}} \right\rbrack}d\;\theta}}{\int{{\exp\left\lbrack {{- 2}\left( \frac{\theta}{\theta_{1/2}} \right)^{2}} \right\rbrack}{\exp\left\lbrack {{- 2}\left( \frac{\theta}{\theta_{1/2}} \right)^{2}} \right\rbrack}d\;\theta}}} & (11)\end{matrix}$

(Note that Equation 11 involves one-dimensional integrals over a singleangle instead of two-dimensional integrals over solid angles. This isbecause the integration over angle in the direction orthogonal to thebeam deviation yields a constant that drops out of the ratio in Equation11.) Equation 11 can be simplified to yield:

$\begin{matrix}{{overlap} = {\exp\left\lbrack {- \left( \frac{2ɛ}{\theta_{1/2}} \right)^{2}} \right\rbrack}} & (12)\end{matrix}$

In summary, one may calculate the relevant deviation angle ε in terms ofknown parameters according to Equation 10. The resulting overlap maythen be calculated according to Equation 12. This gives the ratio ofcross-coupled intensity to self-coupled intensity, assuming that in theself-coupled case there is a perfect waist at the coupler.

There is a very interesting possibility for having a large impact oncross-coupling when near but not quite at the usual configuration ofplacing the emitters one focal length back from L1. If we preciselyplaced the emitters there, then the first term in the denominator ofEquation 10 would be zero, and Equation 10 would reduce to:

$\begin{matrix}{ɛ_{({L_{0} = f_{1}})} = \frac{(d)\left( L_{{cplr}\;\_\;{eff}} \right)}{2\; f_{1}^{2}}} & (13)\end{matrix}$

Substituting Equation 13 into Equation 12 yields:

${overlap} = {{\exp\left\lbrack {{- 4}\left( \frac{d*\left( L_{{cplr}\;\_\;{eff}} \right)}{d^{\prime}*{Zr}} \right)^{2}} \right\rbrack}.}$

Here d′ is the emitter diameter at the near field, and Zr is theRayleigh range of the beam. Thus, to reduce cross coupling thenear-field fill-factor (d/d′) should be high, the optical path lengthbetween the grating and coupler should be long, and the Rayleigh rangeshould be short. Typically the near-field fill-factor is fixed. As anexample, if it is assumed that the WBC system includes 20 diode bars anda transform lens having a focal length of 2000 mm, then the beam size atthe grating is roughly 40 mm (assuming 20 milli-radian full beamdivergence). The Rayleigh range of such a beam (1 μm wavelength anddiffraction limited) is about 160 m. The distance between the gratingand output coupler should be comparable to the Rayleigh range for crosscoupling mitigation. Such length would make the WBC system essentiallyimpractical. However, if the beam is de-magnified by 40× between thegrating and the output coupler the optical path length is shortened by160× or to about 1 m. Further reduction in optical path length may beachieved using larger reduction in beam size. The beam de-magnificationmay be accomplished using various mechanisms such as lenses, prisms, ora combination of both. Careful design must be considered such thatself-coupling of each emitter does not degrade such that the cavitysuffers in performance.

But, if the emitters are slightly off from this position, then the firstterm in the denominator of Equation 10 can actually cancel the secondterm, making the required deviation angle infinite and thecross-coupling overlap zero. Specifically, this happens when:

$\begin{matrix}{L_{0} = {f_{1} + \left( \frac{f_{1}^{2}}{L_{{cplr}\;\_\;{eff}}} \right)}} & (14)\end{matrix}$

In other words, when the effective distance to the coupler L_(cplr) _(_)_(eff) is very large, Equation 14 gives us a potential recipe forpulling the emitters slightly back from the front focus of L1 in orderto destroy cross-coupling.

FIG. 3 illustrates one example of a cross-coupling mitigation system 250illustrated by a box in FIG. 2. Here, optical element 302 may be a lenshaving a focal length F₁ 304. A second optical element 306 may also be alens and have a focal length F₂ 308. The distance between 302 and 306 isexactly or approximately the sum of the focal lengths F₁ and F₂. Asdiscussed previously, it is preferred that the ratio of F₁ over F₂(F₁/F₂) is at least two times or greater. The system 250 may be anafocal telescoping system. In other embodiments, multiple opticalelements may be used wherein the effect of the system still maintainsthe properties of an afocal telescoping system having a large ratio.

In various embodiments, it is desirable to place lens 302 within theRayleigh range of beams being transmitted from a dispersive element(e.g., diffraction grating) while also placing a partially-reflectiveoutput coupler or other reflective surface within the Rayleigh range ofbeams coming out of lens 306. By appropriately placing lenses having aF₁>>F₂ relationship within these positions, an effective system iscreated to reduce and in some cases eliminate any cross-couplingfeedback from entering the non-originating emitter or source.

FIG. 4 illustrates a stabilization system 400 (that may be a portion ofa WBC laser system) in accordance with an embodiment of the presentinvention in which an optical cross-coupling mitigation system (whichmay include or consist essentially of optical elements 410, 420) isutilized in conjunction with a partially reflective output coupler 430engineered to minimize reflections that might result in unwantedfeedback. As shown, the output coupler 430 includes a partiallyreflective beam-receiving portion 434 sized and positioned to receivethe beam from optical element 420. Specifically, beam-receiving portion434 typically has a diameter (or other lateral dimension) approximatelythe same size as the diameter (or other lateral dimension) of the beamit receives. The beam-receiving portion 434, which may be approximatelycentered on a surface of the output coupler 430, is surrounded by anon-reflective portion (or surface) 432 that has a reflectivity of 1% orless to the wavelengths of the received beam. For example, thenon-reflective portion 432 may be coated with an anti-reflection coatingto prevent undesired back-reflection that might result in opticalcross-talk. Thus, any stray light propagating to the output coupler 430outside of the beam-receiving portion 434 will not be reflected back tothe beam emitters of the WBC system. The beam-receiving portion 434 mayprotrude from the remainder of the surface of coupler 430 (i.e., may beelevated with respect to non-reflective portion 432), as shown in FIG.4, or the beam-receiving portion 434 may be approximately coplanar withnon-reflective portion 432.

The beam-receiving portion 434 may have a reflectivity to thewavelengths of the beam of less than approximately 15%, e.g., in therange of approximately 2% to approximately 10%, so as to provide thedesired wavelength stabilization of the beams from the associatedemitters. It will be appreciated that the remainder of the received beamwill pass through the output coupler 430 and be transmitted todownstream optical system components (e.g., an optical fiber or aworkpiece).

FIG. 5 illustrates a stabilization system 500 (which may be a portion ofa WBC laser system) in accordance with an embodiment of the presentinvention in which an optical element 510 (e.g., a cylindrical orspherical lens) simply focuses the beam onto the partially reflectiveoutput coupler 430, and no second collimating optical element is presenttherebetween. In this manner, the output coupler 430 may be utilized ina WBC laser system without the use of an optical cross-couplingmitigation system (e.g., one including or consisting essentially of twoor more optical elements such as lenses).

FIG. 6 illustrates a portion of a WBC laser system 600 in which anoptical element 610 focuses light directly into the core 650 of anoptical fiber that includes one or more features operating as thepartially reflective output coupler. As shown, the optical fiber mayalso have a cladding 640 surrounding the core 650; typically, thecladding 640 has a refractive index less than that of the core 650 suchthat light within the core 650 is confined. The end surface of the core650 may be substantially coplanar with the end surface 642 of thecladding, or the core 650 may protrude slightly from surface 642. Inorder to provide the wavelength stabilization, the end surface of thecore 650 may be partially reflective to the wavelengths of the beam(e.g., between approximately 2% and approximately 10% reflective, orbetween approximately 4% and approximately 10% reflective). In variousembodiments, the partial reflectivity may be provided by a coating onthe end surface of the core.

In various embodiments of the invention, instead of or in addition to apartially reflective coating, a fiber Bragg grating 654 may be providedwithin the core 650 to provide the desired partial reflectivity. Asknown to those of skill in the art, the fiber Bragg grating includes orconsists essentially of a periodic variation of the refractive index ofa portion of the fiber (e.g., within the core 650). The period variationmay be, e.g., on the order of one-half of the wavelength (or one of thewavelengths) of the received beam, and the grating thus induces Fresnelreflection. The wavelength dependence and/or the magnitude of thereflection may be selected by the particular grating pattern and therefractive-index variation therein. In various embodiments, multiplefiber Bragg gratings 654 may be disposed within the core 650, and eachgrating 654 may have a different refractive-index variation and/orwavelength selectivity.

In various embodiments, the surface 642 of the cladding 640 may becoated with an anti-reflective coating so as to prevent any deviatinglight which strays from the beam from reflecting back into adjacentemitters or beams. For example, the surface 642 may be coated so as tohave a reflectivity of less than 1% for the wavelengths of the beam.

FIG. 7 illustrates a wavelength stabilization system 700 (which may be aportion of a WBC laser system) in which an optical element 710 focuseslight into the core 650 of the optical fiber. In system 700, an end cap720 is disposed over and in contact with core 650 (and, in someembodiments, surface 642 of the cladding 640); in some embodiments, theend cap 720 is attached to the optical fiber with an index-matchingmaterial therebetween. In other embodiments, at least a portion of theoptical fiber (e.g., core 650) is directly fused to the end cap 720. Asshown in FIG. 7, the presence of the end cap 720 enables the effectiveinterface between the optical fiber and the oncoming beam (i.e., thepoint at which the beam enters the end cap) to receive the beam when ithas a greater diameter (or width), thereby reducing the power density ofthe beam upon entry to the optical fiber. The presence of the end cap720 may also protect other portions of the optical fiber from heat,moisture, and/or other environmental contaminants.

FIG. 8 illustrates a wavelength stabilization system 800 (which may be aportion of a WBC laser system) in which an optical element 810 focuseslight into the core 650 of the optical fiber. System 800 incorporates amode stripper 820 in order to further increase the purity andtransmission capabilities of the optical fiber with respect to the beam.It will be appreciated that as beams change transmission media thevarious refraction indices and light entrance angles into an opticalfiber may result in a cladding mode, i.e. light traveling within thematerial of the cladding. This cladding mode may be undesirable, as suchlight may result in wavelength distortion and contamination of theprimary beam. As known to those of skill in the art, the mode stripper820 may include, consist essentially of, or consist of a material havinga refractive index no less than (i.e., equal to or greater than) that ofthe cladding 640; in this manner, light that might typically propagateas a cladding mode within the cladding will preferentially enter themode stripper and radiate out of the optical fiber. In variousembodiments, the mode stripper 820 will have a refractive index largerthan that of the cladding 640. As shown in FIG. 8, an index-matchingmaterial 830 may be disposed between the core 650 (or, in someembodiments, the cladding 640) and the mode stripper 820. (As usedherein, the term “index-matching material” refers to a material disposedbetween two other materials and having a refractive index between therefractive indices of the two materials or approximately equal to therefractive index of one or both of the materials.) Although FIG. 8depicts the mode stripper 820 as directly surrounding the core 650, invarious embodiments at least a portion of the cladding 640 is disposedbetween the core 650 and the mode stripper 820.

In any of the aforementioned wavelength stabilization systems it will beappreciated that the beam may be manipulated in various ways via theaddition of optical and/or dispersive elements configured to achieve thedesired beam qualities. For example, optical elements such as gratingsand/or collimators may be present in the WBC system and/or thestabilization system. It will also be appreciated that the partiallyreflective elements may be provided with partially reflective propertiesby any number of means, including but not limited to providing gratings,coatings, etc. so as to achieve the desired transmission and desiredreflection qualities.

The terms and expressions employed herein are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:
 1. A laser system comprising: an array of beamemitters each emitting a beam having a different wavelength; focusingoptics for focusing the beams toward a dispersive element; a dispersiveelement for receiving and dispersing the focused beams, thereby forminga multi-wavelength beam; and an optical fiber for receiving themulti-wavelength beam, the optical fiber comprising (i) a core having,at an end of the optical fiber, a partially reflective outer surface for(a) receiving the multi-wavelength beam, (b) reflecting a first portionthereof back to the array of beam emitters via the dispersive element,wherein the first portion stabilizes each of the beams to itswavelength, and (c) transmitting a second portion thereof as an outputbeam composed of multiple wavelengths, and (ii) surrounding the core, acladding having an outer surface (a) partially defining an end surfaceof the optical fiber along a diameter of the optical fiber and (b)having a reflectivity to the multi-wavelength beam of less than 1%. 2.The laser system of claim 1, wherein a portion of the core protrudesfrom the cladding.
 3. The laser system of claim 1, wherein the opticalfiber is positioned such that, at the partially reflective surface ofthe core, a diameter of the core is not less than a diameter of themulti-wavelength beam.
 4. The laser system of claim 1, furthercomprising an end cap attached to the optical fiber and disposedoptically upstream of the partially reflective surface of the core. 5.The laser system of claim 1, further comprising an anti-reflectivecoating disposed over the cladding of the optical fiber.
 6. The lasersystem of claim 1, further comprising a mode stripper disposed around atleast a portion of the core of the optical fiber.
 7. The laser system ofclaim 1, further comprising a cross-coupling mitigation system forreceiving and transmitting the multi-wavelength beam while reducingcross-coupling thereof.
 8. The laser system of claim 7, wherein thepartially reflecting surface of the core of the optical fiber isdisposed within a Rayleigh range of the multi-wavelength beamtransmitted by the cross-coupling mitigation system.
 9. The laser systemof claim 7, wherein at least a portion of the cross-coupling mitigationsystem is disposed within a Rayleigh range of the multi-wavelength beamtransmitted by the dispersive element.
 10. The laser system of claim 7,wherein the cross-coupling mitigation system comprises an afocaltelescope.
 11. The laser system of claim 7, wherein the cross-couplingmitigation system comprises a first optical element having a first focallength and a second optical element having a second focal length, thefirst optical element being disposed optically upstream of the secondoptical element.
 12. The laser system of claim 11, wherein the firstfocal length is at least two times greater than the second focal length.13. The laser system of claim 11, wherein the first focal length is atleast seven times greater than the second focal length.
 14. The lasersystem of claim 11, wherein each of the first and second opticalelements comprises a lens.
 15. The laser system of claim 11, wherein thefirst optical element is disposed within a Rayleigh range of themulti-wavelength beam transmitted by the dispersive element.
 16. Thelaser system of claim 11, wherein the partially reflecting surface ofthe core of the optical fiber is disposed within a Rayleigh range of themulti-wavelength beam transmitted by the second optical element.
 17. Thelaser system of claim 11, wherein an optical distance between the firstand second optical elements is approximately equal to a sum of the firstand second focal lengths.
 18. The laser system of claim 1, wherein thepartially reflective outer surface of the core has a reflectivity to themulti-wavelength beam of less than approximately 15%.
 19. The lasersystem of claim 1, wherein the partially reflective outer surface of thecore has a reflectivity to the multi-wavelength beam ranging fromapproximately 2% to approximately 10%.
 20. The laser system of claim 1,wherein the partially reflective outer surface of the core has areflectivity to the multi-wavelength beam ranging from approximately 4%to approximately 10%.